An ideal three-hour bulk charge
60 Ah divided by 20 A is 3 hours. The charge rate is 0.2C. This is an arithmetic example, not a recommended setting.
Charge-time estimate 3 h Open in the calculator
Battery charge time calculator
Start with the charge you need to replace. See how much charger current reaches the battery, then allow separately for taper and balancing.
20start
80target
20charger
Battery scale: 0% at left, 100% at right. The bright section adds 60 Ah from the starting level to your target.
20 A charger − 0 A concurrent load, capped at 50 A battery limit
20 A reaches the battery · 0.2 C
3 h constant-current estimate, plus 0 min entered taper allowance. The filled window shows additional charge, not a predicted charging curve.
Charge-time estimate
3h
20% to 80% of 100 Ah.
Check the values to see a result.
- Current reaching the battery
- 20 A
- Charge rate
- 0.2 C
- Charge to store
- 60 Ah
Constant-current estimate plus your separate taper/balancing allowance. Use the manufacturer’s current, voltage and temperature limits.
How it’s calculated
- Net battery current = min(charger − load, battery limit), at least zero=min(20 − 0, 50)=20 A
- Charge required = capacity × change in SOC=100 × 0.6=60 Ah
- Constant-current time = charge required ÷ (net current × Ah efficiency)=60 ÷ (20 × 1)=3 h
Time = capacity × SOC change ÷ (net battery current × Ah efficiency) + extra time
The estimated time is 3 hours, including 0 minutes of extra time when charge is required.
Tip: A 20 A charger does not put 20 A into the battery while it also supplies a 5 A load. Charging headroom is 15 A before the battery limit.
Follow the current to the battery
Charging time starts with the amp-hours missing from the target state of charge. A 100 Ah battery moving from 20% to 80% needs to store 60 Ah. A 20 A charger would replace that in three hours only if all its current reaches the battery, amp-hour efficiency is 100%, and current does not taper.
The calculator exposes each assumption. It subtracts a concurrent DC load, applies the battery current limit, and adds your separate allowance for absorption, taper or balancing. It does not infer a safe charging profile from the battery capacity.
Find the charge deficit
Required Ah = rated Ah × (target SOC − starting SOC) ÷ 100
Use the same capacity basis as the SOC estimate. Ageing and temperature can change available capacity, and a voltage-only SOC estimate under load can be misleading.
Apply both limits
Net A = max(0, min(charger A − load A, battery limit A))
A load supplied by the same charger consumes headroom before charging. The battery limit is a separate ceiling on current entering the battery. If no headroom remains, a finite charging time is not available.
Separate charge efficiency from energy efficiency
Constant-current hours = required Ah ÷ (net A × Ah efficiency)
Amp-hour efficiency compares charge stored with charge entering. It does not account for the different charging and discharging voltages, so it is not interchangeable with round-trip kWh efficiency.
Add the non-constant-current period
Estimated hours = constant-current hours + extra minutes ÷ 60
Use a manufacturer charging curve or observed session to estimate extra time. A generic percentage does not reproduce a lead-acid absorption stage or lithium cell balancing. The extra time is zero when the battery is already at target.
Battery charge time examples
A running load and an extra allowance
15 A reaches the battery. 60 ÷ (15 × 0.98) is about 4.082 hours; the entered half-hour allowance gives about 4.582 hours.
Charge-time estimate 4.582 h Open in the calculator
Questions about Battery charge time
What does 0.2C mean?
Current equal to 0.2 times the amp-hour capacity: 20 A for a 100 Ah battery. It describes the rate, not whether that rate is safe for a particular battery.
Why is charging to 100% often slower?
Many charging profiles reduce current as the voltage limit is reached. Absorption or cell balancing can extend the session. The calculator needs an explicit allowance because it does not model that curve.
Can I enter an AC charger input current?
No. Enter charger output current at the battery DC voltage. AC input current does not directly state battery charging current.
Limits of this result
- Use amp-hour capacity and DC current at the same battery voltage. This is not an AC charger input-power calculation.
- Amp-hour efficiency is not energy or round-trip efficiency. A chemistry-specific charging curve may give a different result.
- The added time is your assumption, not a predicted absorption or balancing duration. Current taper can begin below the target SOC.
- Check the manufacturer’s voltage, temperature, current and charging profile. No C-rate shown here is a recommended charge limit.